Personality, like they said, determines how people think of you. If you constantly be nice to people, people will have a great impression of you (But a minority of them might labeled you as someone who can be taken advantage of). Be mean to your friend just once, and you'll be forever condemned in their eyes (But most of them will labeled you as a selfish friend). This is not at all unreasonable, as your friends are always right (sort of). You are not always wrong though, but what you see as something quite reasonable and fair, your friend may not see things the same way. That's OK though, as your friend is right, and you're wrong, mostly.
From my point of view, one can certainly change his personality, and thus, behavior (People always do). However, when you change, you risk offending your friends. For example, you used to be nice to people, you always helped them with their responsibilities, at the expense of your own time, never mind you have to stay overnight to do it, risking the fact that you may not have enough time to complete your own work. One day, when you decided that it is a foolish thing to do, and you break the behavior. The problem with that is, people are not expected to have a radical (It's considered quite radical, actually) change of behavior. If you do, people could come up with various wild conspiracy theories about why you don't help them and most of the time the conclusion that they have come up with will not be in your favor. Precisely why, is totally beyond me. Perhaps that is how the human brain works. They're designed to be suspicious and always think of the worst scenario (Though I was told that girls has an extreme version of this ability). This ability, is useful in the sense that, it allows people to prepare for the worst, such as deserting friends that has lost its usefulness. Not everyone do that, but some do (though some that do, without realizing it).
Now, what happened when you have a change of behavior that do not benefit your friends? Various things happens. From now on, you will be suspicious of everything. Even things that are not of your fault. You are of course expected to live with that, since you are the one that initiated that change of behavior. And you can't blame them, since in theory, your brain works the same way, just that in this case, you are the one that has changed, not your friend.
In spite of all this, should you care about all these complexities? Probably not. All you have to do, is determine what is the best you can do to yourself first. This is not selfishness, this is called self-interestness. Unless of course, by valuing yourself more than others you harm a great number of people. In which case you should probably sacrifice yourself like Jesus does. Don't worry, as you would surely go to heaven (The bible says so).
I for one has got tired of trying to be someone I'm not. I'm not a stunt man. I don't pull stunts in real life. Stunts like helping people finish his assignment when I have more important stuff to attend to. It's not a lame excuse to bring harm to my friends. Rather, it's a hard conclusion I've come to after I've done a full reality check on myself. And the reality is indeed a cruel one. The question is, is the world becoming more cruel and bleak? I think not. It's merely becoming more matured. Likewise, a matured person tends to not take things likely. They think twice before jumping into conclusions.
